Problem using QtCreator as a general C/C++ IDE (finding symbols)

I love QtCreator. I use it very effectively with Qt projects that I've been a part of. However, I would like to be able to use the code navigation and completion features in a non-Qt project.

I have a project that I can successfully create using the process:

File->New Project->Import Project->Import Existing Project
I name the project and select the location of the existing code.

That is fine, I get completion and following of symbols for that small code base. This project also uses a library from another company, and source for that is in a completely different source tree. What I've tried is:

Project->right-click->Add Existing Directory
Add the source directory

These steps update my <project>.files and <project>.includes files, but I cannot then use F2 (follow symbol) to get to the declarations of certain structures in the newly added source directory.

It seems this should be possible. Am I doing something wrong or am I trying to do something that won't work? I ask because I can set this up in Eclipse CDT and I consider QtCreator to be a superior IDE for C/C++.